This invention relates to a high frequency amplifier for a very high frequency band such as GHz, and more particularly, to a high frequency amplifier with a gain varying function.
With an increase in demand for mobile communication, there is a strong demand for a reduction in the size and cost of cellular phones and other type mobile communication apparatuses. To meet the request, research has been made into integrating as many functions as possible in high frequency circuits, such as high frequency amplifiers, incorporated in mobile communication apparatuses, to decrease the number of chips used in the apparatuses.
In general, high frequency amplifiers have different gains due to differences in characteristics therebetween or changes in temperature. Further, modulators, for example, which are each located on the signal source side of the high frequency amplifier, have different output amplitudes. To deal with the gain variations of high frequency amplifiers or the amplitude variations of input signals, a gain varying function is provided in many high frequency amplifiers by incorporating therein a variable gain amplifier or a variable attenuator.
To impart a gain varying function to a high frequency amplifier, it is required, particularly in digital mobile communication apparatuses, that the linear characteristic of the amplifier be prevented from a great change even when its gain is varied, in order to maintain the output of the amplifier at a value suitable for the specifications of the apparatus.
To realize the gain varying function of the high frequency amplifier, several methods are proposed.
The first one is to use a variable gain amplifier constituted of two FETs connected in cascade, or of a dual gate FET.
FIG. 1 shows an example of the former. As is shown in FIG. 1, FETs Q11 and Q12 are cascade-connected. Specifically, the gate of the FET Q11 is connected to a high frequency input terminal RFin, and the drain of the FET Q12 to a high frequency output terminal RFout.
The gate of the FET Q11 is connected to a gate bias power Vg via a resistor R11, and the gate of the FET Q12 to a gain control power Vc via a resistor R12.
Although the variable gain amplifier can be varied in gain by controlling the voltage of the gain control power Vc, it does not have an excellent linear characteristic.
More specifically, if cascade-connected FETs or dual gate FETs are used in cellular phones, whose power voltage has been reduced so far and which is now demanded to operate at a low power of, for example, 3 V or less, it is difficult to obtain the drain-source voltage Vds of each FET at a sufficient value since the two FETs are connected substantially in series to the power voltage. As a result, the output waveform of the structure is principally liable to be distorted.
Thus, it is difficult in the conventional art to satisfy the linear characteristic demanded in a digital mobile communication apparatus whose power voltage has been more and more reduced.
The second method for imparting a gain varying function to the high frequency amplifier is to cause a high frequency signal to bypass, using a variable attenuator which includes FETs.
FIG. 2 shows an example of the attenuator. As is shown in FIG. 2, a bypass FET Q21 has a drain connected to a main signal line between a high frequency input terminal RFin and a high frequency output terminal RFout, and a source connected to a bias power Vd and grounded via a capacitor C21.
Moreover, a resistor 21 is connected between the drain and source of the FET Q21, and a gain control power Vg is connected to the gate of the FET Q21 via a resistor R22.
Since in the above structure, the amount of bypass can be varied by varying the voltage of the gain control power Vg (i.e. the gate potential of the FET Q21), thereby varying the drain-source resistance of the FET Q21, the attenuation amount between the high frequency input and output terminals RFin and RFout can be varied.
The variable attenuator is located between an amplifying element and an output matching circuit, which are incorporated in the high frequency amplifier, or between stages of a multi-stage amplifier. In this structure, however, a signal which is amplified by the amplifying element and hence has a large voltage amplitude is made to bypass. Therefore, the output is liable to be distorted because of the non-linear characteristic of the bias FET Q21. Furthermore, when the amount of attenuation is large, the impedance of the bypass FET Q21 which is viewed from the drain side is low, and hence the load line of the amplifying element sharply inclines as compared with a case where the attenuation amount is small. Accordingly, the output is often distorted depending upon current level.
In addition, a variable attenuator suitable for varying the gain of a high frequency amplifier is also proposed in the form of a structure in which a first FET is connected parallel to the input/output terminal side resistance of a T-type attenuator or a .pi.-type attenuator, and a second FET is connected in series to the ground side resistance of it, the attenuation amount being changed with a predetermined impedance kept, by complementarily turning on and off the first and second FETS.
This type attenuator, however, requires a great number of elements and a control circuit for performing on/off control of a plurality of FETs. Accordingly, the overall circuit is inevitably complicated, with the result that the chip area of each product increases or the product yield decreases.
On the other hand, to reduce the chip area, an element formed by a special process, such as an Octal gate FET or a BST capacitor, must be employed, as is known from the T-type attenuator described in Miyatuji.k., and Ueda,D., "A Low Distortion GaAs Variable Attenuator IC for Digital Mobile Communication System", JSSCC Digest. pp. 42-43, February, 1995.
As described above, if in the prior art, a gain varying function is imparted to a high frequency amplifier, the linear characteristic of the amplifier may degrade, or the circuit structure complicate.